1. Field of the Invention
This invention relates to heat-resistant heavy-duty components, e.g. a disk, wheel, etc. of gas turbines for aircrafts and generators, and to a method of fabricating the same wherein dissimilar alloys are junctioned. More particularly, it relates to a method of fabricating a turbine disk, etc. by superplasticity forging from two or more kinds of dissimilar alloys having different which method allows enhancing the reliability of the junction interface between the alloys, and to the turbine disk, thus obtained.
2. Statement of Related Art
In gas turbines, higher temperatures have been achieved by an enhancement in the thermal efficiency and rise in their power output but which requires an increase in the number of revolutions more and more. In order to meet this demand, an enhancement of materials in heat-resistant strength and fatigue characteristics is necessary.
In the case of a gas turbine disk and gas turbine wheel, different characteristics are required between the hub and rim portions. Centrifugal force due to high speed revolution is higher in the hub portion than in the rim portion and, consequently, high tensile strength and fatigue strength that are endurable to higher centrifugal force are required more in the hub portion. Conversely, however, the temperature becomes higher from the hub portion toward the rim portion where creep strength is required rather than tensile strength and fatigue strength. From the standpoint of microstructure of materials, a fine structure is necessary more in the hub portion, whereas a coarse crystal grain structure is necessary more in the rim portion. Thus, characteristics required for a disk or wheel vary depending on its portions.
With a view toward optimizing performances of a gas turbine disk, wheel, etc., a variety of methods have been proposed. Typical methods are exemplified as follows:
(1) Method for enhancing creep property of the rim portion by unidirectional recrystallization heat treatment. In a disk solidified by hot isostatic pressing (hereinafter abbreviated as "HIP") from alloy powders, temperature distribution from the rim portion to the hub portion is controlled so that unidirectional recrystallization coarsening from the rim toward the hub may be performed.
(2) Method of subjecting only the hub portion to forging. Reversely to (1) above, in order to enhance tensile property and fatigue property of the hub portion, a powder HIP material only of the hub portion is processed and recrystallized to a fine structure.
(3) Method of fabricating a turbine wheel by diffusion-junctioning a cast ring of blade and a HIP disk. A ring including the rim portion and blade is fabricated by casting to impart a creep property to the ring as a coarse crystal grain structure while a disk as a hub portion is fabricated by HIP to make a fine structure having a high tensile strength and finally, both are diffusion-junctioned by HIP.
(4) Method of HIP simultaneously different kinds of alloy powders. Two kinds of alloys having different characteristics are disposed in respective portions of a disk so that requisite characteristics may be imparted to the respective portions and, simultaneously, subjected to HIP treatment.
(5) Method of diffusion-junctioning a HIP solidified material with a different kind of alloy powder by HIP. To a rim portion preliminarily treated by HIP, a different kind of alloy powder is diffusion-junctioned by HIP to fabricate a hub portion. Here, the HIP temperature in fabricating the hub portion is generally made lower than in the case of the rim portion.
The foregoing methods, however, have problems and defects which follow. More specifically, the methods of (3) to (5) above availing themselves of diffusion junction by HIP are more advantageous than the methods of (1), (2) above in that the costs of manufacture are cheaper, but the methods (3) and (5) are not satsifactory with respect to reliability of the diffusion junction interfaces. Particularly, the method (5) in which the different alloy powder is diffusion-junctioned to the HIP solidified material has not yet provided any satisfactory joint. In the method (4) wherein different alloy powders are subjected to HIP simultaneously, intermingling of them occurs and consequently, junction boundary is difficult to control and devoid of reliability.